The chief prosecutor of the International Criminal Court, Luis Moreno-Ocampo, is set to accuse President Omar al-Bashir of Sudan of war crimes tomorrow.

Bashir will be the first serving head of state - and the first Muslim leader - to face prosecution by the ICC. In his 19 years in power, Bashir has presided over a regime marked by recurrent massacre and savage repression. His ruling security clique is weak but united by fear of what may happen if it loses power. The country's democratic forces are fragmented, and after decades of struggle and confrontation have realised that the only way out of their crisis is a compromise power-sharing deal.

Some commentators use the word 'evil' to describe Bashir. This is a simplification. Bashir is a pragmatist who responds to incentives. He negotiated an end to the 20-year civil war with marginalised southerners when convinced of the benefits of peace. But he is also a proud and stubborn man, who responds to affronts to his dignity with rage. When humiliated, he lashes out. Today those closest to him recognise the signs of suppressed fury. 'We cannot say how he will react,' they warn ominously.

Millions of Sudanese are hostages to Bashir's calculation and mood. The peace agreement that ended the war in the south hangs by a thread. Five years after rebels took up arms in a separate conflict, in Darfur in western Sudan, one in every three inhabitants of the region is dependent on international aid. Yet the international community has no contingency plan for what to do if Bashir responds in character. Every instrument short of military force has been used to try to make his government bend to the international will.

Ocampo's announcement tomorrow will be the biggest shot yet. But once fired it will be a blow to impunity only if Bashir surrenders, which is impossible; is arrested, which is difficult; or if the regime crumbles, which is unlikely given the weakness of the political opposition and the dismemberment of Sudan's once vibrant civil society. If Bashir lives to fight another day, unrepentant and unreformed, indicting him will be a blow for impunity. Tomorrow, UN Secretary-General Ban Ki-Moon's dream - that 'justice and peace are indivisible' - will turn into a nightmare. It will be tested to destruction - possibly the destruction of Sudan itself. The immediate dangers are easy to foresee. The very people the ICC seeks to defend - the survivors of the Darfur war - are the most vulnerable to whatever steps the regime takes in its fightback.

The UN peacekeeping force in Darfur is already almost at a standstill. A few more restrictions - or deaths - would paralyse it. Humanitarian aid that feeds two million displaced people is dependent on a UN airlift that can be choked off at any moment. Popular demonstrations of support for the ICC could be met with lethal force, prompting a response from the armed rebel supporters who control many of the displaced camps.

The ICC was established 10 years ago this month when 120 states signed the Treaty of Rome, hoping to bring justice to victims of the world's worst human rights violations. It began operations five years ago and has opened four investigations, all of them in Africa: the Democratic Republic of Congo, northern Uganda, the Central African Republic and Darfur. Twelve arrest warrants have been issued, including two for war crimes and crimes against humanity allegedly committed in Darfur by the Minister of State for Humanitarian Affairs Ahmed Haroun and the government-backed militia leader Ali Kushayb.

Neither man has been handed over to the ICC. Neither has been marginalised within the regime. Bashir, who sees the ICC as only one manifestation of an international conspiracy against him, has sworn that he will never surrender any Sudanese national and has promised that Darfur will become a 'graveyard' for the international forces he believes could enforce arrest warrants. As organised hysteria mounts, UN peacekeepers might not be the only targets.

Relief workers could be under threat, too.

On the streets of Khartoum and in the savannas of Darfur, the supporters and adversaries of the ICC are already squaring up. Unamid, the Darfur peacekeeping force, is retrenching, predictably focusing on protecting its own men, and is not expected to intervene to protect civilians if war is reignited. International relief workers are lowering their profile: many have already left Darfur; others are hunkering down. Darfur's rebels will be emboldened, perhaps sufficiently to launch new military offensives to which the regime will respond as it has responded ever since it seized power - with massive violations of human rights.

Who then will protect the victims and the vulnerable? Who will police the ceasefire between north and south? What could compel Bashir, hunted by the ICC, to comply with his promise to hold national elections that could unseat him in 2009? Should Bashir respond in character - with defiance - he will have called the bluff of the international community. Each time the UN Security Council meets on Sudan, it will demand that Bashir surrender himself to the ICC, and each time he refuses to do so, it will need to ratchet up the pressure.

The price of sanctions, isolation and conflict will be borne by the Sudanese people. Those who support indicting Bashir are comparing him with Liberia's Charles Taylor and former Yugoslavia's Slobodan Milosevic, who were themselves indicted while serving heads of state. They argue that the indictments moved those countries towards peace. But in both those cases the West was enacting a plan for regime change. In Sudan, by contrast, it is supporting a negotiated transition to peace and democracy with mechanisms for civilian protection.

The chief prosecutor's timing could not be worse. There has been more movement on the north-south Comprehensive Peace Agreement, which provides a blueprint for the democratisation of Sudan, in the past two months than in the past three and a half years; a deal has been signed to end a dispute over the flashpoint oil-rich region of Abyei; and an election law has been passed. In the interests of the people of Sudan, Ocampo should reconsider. It is not too late.

· Julie Flint and Alex de Waal are the co-authors of Darfur: A New History of a Long War, published by Zed Books in May.